1. Field of the Invention
This invention relates to a heat-developable photosensitive member for forming electrostatic printing masters, and more particularly, to an improved heat-developable photosensitive member for forming electrostatic printing masters, containing an organic silver salt.
2. Description of the Prior Art
Many printing methods are known. Among them, electrostatic printing methods belong to a special class. The principle of ordinary printing is based on selective application of ink onto the surface of a printing master due to uneven surface condition of the printing master or difference in solvent affinity, and transfer of the ink to a paper by a pressing action. On the other hand, in the electrostatic printing, the ink is replaced by a heat-fixable toner which is electrostatically adhered to a printing master, then transferred to and fixed on an image-receiving sheet, e.g., paper. While the ordinary printing has such as advantage that the ink is placed uniformly and firmly on the printing master to enable a large number of sheets of paper to be printed at high speed, it has such a disadvantage that the ink is liable to adhere to portions of printing paper other than those to be printed. On the other hand, in the electrostatic printing methods, the toner can be adhered electrostatically so that firmness and uniformity of adhesion are heartily known dependent upon electrostatic "contrast" which is difficult to achieve, hence the method is not suitable for high speed printing, although staining of the printing paper as mentioned above is not so much problem as in ordinary printing. In view of the stated disadvantages, electrostatic printing has not been practically used as a clean printing method. In other words, electrostatic printing is poorer than conventional printing methods as to providing uniform and clear print in large number of sheets. For example, a representative electrostatic printing master which has been known is composed of a conductive support and an insulating image overlying the conductive support, or composed of an insulating support and a conductive image overlying on the insulating support. The image may be produced by applying an insulating or a conductive lacquer in the form of an image pattern onto the support, or by coating a photosensitive lacquer on a support, imagewise exposing and selectively removing the exposed or unexposed portions by etching. Such electrostatic printing masters have various drawbacks. For example, when it is used in the conventional electrostatic printing process sharpness of the printed image and durability of the master are usually poor. Such electrostatic printing process includes a charging step for forming an electrostatic image by selectively retaining electric charge at image portions (the image portions are insulating), a developing step, in which a toner having a polarity opposite to that of the image portions is applied, and a transfer step, in which the toner image is transferred to a receiving sheet. For example, the known electrostatic printing master has images formed on its uneven surface, and such uneven surface is damaged by mechanical abrasion during the printing process to cause irregular charging, so that durability of the master is very low. Further more, it is very difficult to obtain a high resolving power with such uneven surface type master and also technically difficult to obtain a print having high resolution. Additionally, it is difficult to obtain images of half tone or gradation with such uneven surface type master.
U.S. Ser. No. 599,061 filed July 25, 1975 discloses a novel electrostatic printing master capable of solvent the disadvantages of known electrostatic printing masters. The novel electrostatic printing master has a layer composed of an electrically insulating medium having an electric resistance sufficient to retain electrostatic charge and silver images carried therein and therefore, the surface of the master is not uneven, but smooth. Such structure of the novel master results in substantially no damage of images caused by mechanical abrasion upon printing, high durability, high resolving power due to high resolution and continuous gradation of silver image itself, and an optical density changeable depending upon an optional continuous gradation.
Usually such electrostatic printing masters having a layer containing silver images as mentioned above can be produced by imagewise exposing a silver salt photosensitive member and then developing. In particular, photosensitive members containing an organic silver salt are preferable because the silver images can be treated in a dry system and the procedure is very simple.
Such photosensitive members are so-called heat-developable photosensitive members, for example, those disclosed in U.S. Pat. Nos. 3,457,075, 3,531,286 and 3,589,903. However, those known heat-developable photosensitive members are heat-developable photosensitive members for producing a visible image as a final image directly on said members, that is, for direct copying, but do not have sufficient characteristics necessary for an electrostatic printing master. Therefore, known heat-developable photosensitive members are practically not suitable for heat-developable photosensitive members for forming electrostatic printing masters of U.S. Ser. No. 599,061.
Printing process using an electrostatic printing master is carried out, for example, by charging the master with a corona discharger to convert electric resistance patterns to electrostatic patterns, developing said electrostatic patterns with toner particles, and transferring the resulting toner images to an image receiving sheet such as paper. Therefore, an electrostatic printing master should have various characteristics such as high acceptance potential and high electrostatic charge retentivity at a portion to be electrostatically charged (a portion of a relatively high resistance), low background potential, high electrostatic potential contrast, high mechanical, electrostatic and repeating durability, excellent property of development and good cleaning property, the final image formed on a receiving sheet by transferring being sharp, high electric fatigue resistance, and high mechanical strength. In addition, there are required that the electrostatic printing master is easily and simply formed within a short time, the heat-developable photosensitive member is easily produced, the master is inexpensive, can be easily transported without any particular procedure, and can be easily distributed.